Multi Channel Interface For Powerbook, Newbie seeking advice on best option Options

[dunks]

Have new powerbook. Use garageband 2. May upgrade to logic express in future. Would like interface to allow multi track recording. EG: jazz quartet: Mic up guitar, bass and keyboard amps and one mic for the drummer. Record each instrument on seperate track in garageband during live performance and be able to mixdown later after the gig. What would people suggest - cheaper the better!